Tetracyanobutadiene (TCBD) functionalized benzothiadiazole derivatives: effect of donor strength on the [2+2] cycloaddition-retroelectrocyclization reaction

In order to explore the selectivity of the [2+2] cycloaddition-retroelectrocyclization reaction in the presence of different donor groups, a series of unsymmetrical and symmetrical mono/di 1,1,4,4-tetracyanobutadiene (TCBD) substituted benzothiadiazoles (BTDs) 2a-2g were synthesized. 2a-2e exhibit monosubstitution, whereas 2f and 2g show disubstitution of TCBD in the presence of an excess amount of tetracyanoethylene (TCNE). The photophysical, electrochemical and thermal properties of the BTDs 2a-2g were systematically studied. The absorption spectra of BTDs 2a-2g exhibit a strong electronic absorption band in the visible region of the electromagnetic spectrum, which corresponds to the intramolecular charge transfer (ICT). The TGA data reveal that BTD 2e exhibits better thermal stability compared to the other BTDs. The electrochemical studies of BTDs 2a-2g exhibit a strong electronic communication in 2f and 2g due to the incorporation of an additional TCBD acceptor. The computational studies reveal that the incorporation of an additional TCBD unit and systematic variation of terminal donor groups are the key factors to tune the HOMO-LUMO energy level. The structure of the BTD 2c was confirmed by single crystal X-ray diffraction analysis.